The present invention relates to printing with a drop-on-demand ("DOD") ink jet print head wherein ink drops are generated utilizing a drive signal that controls the operation of the ink jet print head to reduce rectified diffusion. Rectified diffusion is the growth of air bubbles dissolved in the ink from the repeated application of pressure pulses, at pressures below ambient pressure, to ink residing within the ink pressure chamber of the ink jet print head. Rectified diffusion results in print quality degradation over time. By controlling the operation of the ink jet print head, the drive signal may also simultaneously reduce rectified diffusion and enhance the consistency of drop flight time from the ink jet print head to print media over a wide range of drop ejection or drop repetition rates.
Ink jet printers, and in particular DOD ink jet printers having ink jet print heads with acoustic drivers for ink drop formation, are well known in the art. The principle behind an ink jet print head of this type is the generation of a pressure wave in and the resultant subsequent emission of ink droplets from an ink pressure chamber through a nozzle orifice or ink drop ejection orifice outlet. A wide variety of acoustic drivers is employed in ink jet print heads of this type. For example, the drivers may consist of a pressure transducer formed by a piezoelectric ceramic material bonded to a thin diaphragm. In response to an applied voltage, the piezoelectric ceramic material deforms and causes the diaphragm to displace ink in the ink pressure chamber, which displacement results in a pressure wave and the flow of ink through one or more nozzles.
Piezoelectric ceramic drivers may be of any suitable shape such as circular, polygonal, cylindrical, and annular-cylindrical. In addition, piezoelectric ceramic drivers may be operated in various modes of deflection, such as in the bending mode, shear mode, and longitudinal mode. Other types of acoustic drivers for generating pressure waves in ink include heater-bubble source drivers (so-called bubble or thermal ink jet print heads) and electromagnet-solenoid drivers. In general, it is desirable in an ink jet print head to employ a geometry that permits multiple nozzles to be positioned in a densely packed array, with each nozzle being driven by an associated acoustic driver.
U.S. Pat. No. 4,523,200 to Howkins describes one approach to operating an ink jet print head with the purpose of achieving high velocity ink drops free of satellites and orifice puddling and providing stabilized ink jet print head operation. In this approach, an electromechanical transducer is coupled to an ink chamber and is driven by a composite signal including independent successive first and second electrical pulses of opposite polarity in one case and sometimes separated by a time delay. The first electrical pulse is an ejection pulse with a pulse width which is substantially greater than that of the second pulse. The illustrated second pulse in the case where the pulses are of opposite polarity has an exponentially decaying trailing edge. The application of the first pulse causes a rapid contraction of the ink chamber of the ink jet print head and initiates the ejection of an ink drop from the associated orifice. The application of the second pulse causes rapid expansion of the ink chamber and produces early break-off of an ink drop from the orifice. There is no suggestion in this reference of controlling the position of an ink meniscus before drop ejection; therefore, problems in printing uniformly at high drop repetition rates would be expected.
U.S. Pat. No. 4,563,689 to Murakami et al. discloses an approach for operating an ink jet print head with the purpose of achieving different size drops on print media. In this approach, a preceding pulse is applied to an electromechanical transducer prior to a main pulse. The preceding pulse is described as a voltage pulse that is applied to a piezoelectric transducer in order to oscillate ink in the nozzle. The energy contained in the voltage pulse is below the threshold necessary to eject a drop. The preceding pulse controls the position of the ink meniscus in the nozzle and thereby the ink drop size. In FIGS. 4 and 8 of Murakami et al., the preceding and main pulses are of the same polarity, but in FIGS. 9 and 11, these pulses are of opposite polarity. Murakami et al. also mentions that the typical delay time between the start of the preceding pulse to the start of the main pulse is on the order of 500 microseconds. Consequently, in this approach, drop ejection would be limited to relatively low repetition rates.
These prior art methods for operating ink jet print heads have difficulty achieving uniformly high print quality at high printing rates. Another potential problem associated with ink jet print heads is degradation in printing quality resulting from rectified diffusion. Rectified diffusion occurs when air bubbles dissolved in the ink grow from the repeated application of pressure waves or pulses, at pressures below ambient pressure, to ink residing within the ink pressure chamber of the ink jet print head. After a certain period of time, called the "onset-period," the printing quality degrades from continuously operating the ink jet print head in this manner. The onset-period depends on the drop repetition rate, and, prior to the initiation of continuous ink jet print head operation, on the amount of air dissolved in the ink, the ink viscosity, the ink density, the diffusivity of air in the ink, and the radii of the air bubbles dissolved in the ink. A need exists for a method of operating an ink jet print head that extends or eliminates the onset-period. A need also exists for a method that extends or eliminates the onset-period while simultaneously achieving high print quality at high printing rates.